1. Field of the Invention
The present invention relates to a light-emitting element driving circuit.
2. Description of the Related Art
A light-emitting element used as a light source requires an APC (Auto Power Control) circuit used to maintain a constant emitted light amount in, for example, use application of an optical encoder or the like. As the conventional APC circuit, for example, a light-emitting element driving circuit, which monitors the emitted light amount of a light-emitting element and controls driving of the light-emitting element so as to approach an emitted light amount as a target value, is known. In order to shorten a start-up time of this light-emitting element driving circuit and to speed up an APC operation, it is required to control a potential of a driving control node of the light-emitting element within a short period of time. According to Japanese Patent Laid-Open No. 10-93171, prior to the APC operation, a capacitor connected to the driving control node of the light-emitting element is charged in advance to a predetermined voltage, and only a difference from a reference voltage is charged/discharged at the time of the APC operation, thus speeding up the APC operation. This can be attained using an operational amplifier as an analog buffer before the APC operation, and using it as a comparator at the time of the APC operation.
With a technique disclosed in Japanese Patent Laid-Open No. 10-93171, the single operational amplifier is used in both charging operations before and at the time of the APC operation, and its charging speed does not exceed the drivability of the operational amplifier. Therefore, speeding-up can be achieved at the time of the APC operation, but a charging time of the capacitor is required before the APC operation, thus requiring a considerably long start-up time. However, it is desirable for the light-emitting element driving circuit to be quickly started up and to execute a high-speed APC operation.